Method for device-to-device (D2D) operation performed by terminal in wireless communication system and terminal using the method

ABSTRACT

A method for device-to-device (D2D) operation performed by a terminal in a wireless communication system, and a terminal using the method are provided. The method comprises: receiving system information including reception resource pool information; and receiving a D2D signal by using resources indicated by the reception resource pool information, wherein the reception resource pool information indicates resources for which receiving the D2D signal is allowed during a radio resource control (RRC) idle state and an RRC connected state.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/125,556, filed on Sep. 12, 2016, now U.S. Pat. No. 10,349,389, whichis the National Stage filing under 35 U.S.C. 371 of InternationalApplication No. PCT/KR2015/002771, filed on Mar. 20, 2015, which claimsthe benefit of U.S. Provisional Application No. 61/968,341, filed onMar. 20, 2014, the contents of which are all hereby incorporated byreference herein in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to wireless communications, and moreparticularly, to a method for a device-to-device (D2D) operationperformed by a terminal in a wireless communication system, and theterminal using the method.

Related Art

In International Telecommunication Union Radio communication sector(ITU-R), a standardization task for International MobileTelecommunication (IMT)-Advanced, that is, the next-generation mobilecommunication system since the third generation, is in progress.IMT-Advanced sets its goal to support Internet Protocol (IP)-basedmultimedia services at a data transfer rate of 1 Gbps in the stop andslow-speed moving state and of 100 Mbps in the fast-speed moving state.

For example, 3^(rd) Generation Partnership Project (3GPP) is a systemstandard to satisfy the requirements of IMT-Advanced and is preparingfor LTE-Advanced improved from Long Term Evolution (LTE) based onOrthogonal Frequency Division Multiple Access (OFDMA)/SingleCarrier-Frequency Division Multiple Access (SC-FDMA) transmissionschemes. LTE-Advanced is one of strong candidates for IMT-Advanced.

There is a growing interest in a Device-to-Device (D22) technology inwhich devices perform direct communication. In particular, D2D has beenin the spotlight as a communication technology for a public safetynetwork. A commercial communication network is rapidly changing to LTE,but the current public safety network is basically based on the 2Gtechnology in terms of a collision problem with existing communicationstandards and a cost. Such a technology gap and a need for improvedservices are leading to efforts to improve the public safety network.

The public safety network has higher service requirements (reliabilityand security) than the commercial communication network. In particular,if coverage of cellular communication is not affected or available, thepublic safety network also requires direct communication betweendevices, that is, D2D operation.

D2D operation may have various advantages in that it is communicationbetween devices in proximity. For example, D2D UE has a high transferrate and a low delay and may perform data communication. Furthermore, inD2D operation, traffic concentrated on a base station can bedistributed. If D2D UE plays the role of a relay, it may also play therole of extending coverage of a base station.

Meanwhile, a network may broadcast the information related to the D2Doperation in a specific cell, which notifies the resources that are ableto receive D2D signals. It is required to regulate how to operate when aterminal receives the information.

SUMMARY OF THE INVENTION

The present invention provides a method for a device-to-device (D2D)operation performed by a terminal in a wireless communication system,and the terminal using the method.

In one aspect, provided is a method for a device-to-device (D2D)operation performed by a user equipment (UE) in a wireless communicationsystem. The method includes receiving system information includingreception resource pool information; and receiving a D2D signal usingresources indicated by the reception resource pool information. Thereception resource pool information indicates resources in whichreception of the D2D signal is allowed in a radio resource control (RRC)idle state and an RRC connected state.

The system information may be broadcasted.

The D2D signal may be a signal for a D2D communication or a signal for aD2D discovery.

The UE may receive the system information including the receptionresource pool information in the RRC idle state.

The UE may receive the D2D signal using the resources indicated by thereception resource pool information, even in the case that the UE isswitched from the RRC idle state to the RRC connected state.

The resources indicated by the reception resource pool information maybe used for another UE to receive a D2D signal transmitted in mode 1 ora D2D signal transmitted in mode 2.

Mode 1 may be a mode that a network is scheduling a resource for a D2Dsignal transmission of another UE, and mode 2 may be a mode that anotherUE selects a specific resource for a D2D signal transmission within aresource pool.

In another aspect, provided is a user equipment (UE) for performing adevice-to-device (D2D) operation in a wireless communication system. TheUE includes a radio frequency (RF) unit configured to transmit orreceive a radio signal and a processor operatively connected to the RFunit. The processor is configured to perform receiving systeminformation including reception resource pool information and receivinga D2D signal using resources indicated by the reception resource poolinformation. The reception resource pool information indicates resourcesin which reception of the D2D signal is allowed in a radio resourcecontrol (RRC) idle state and an RRC connected state.

According to the present invention, a terminal may receive the D2Dreception resource pool information broadcasted by a network. Theterminal may receive D2D signals using the resource indicated by the D2Dreception resource pool information in both of the RRC idle state andthe RRC connected state. The method based on the present invention hasan advantage in the aspect of signaling and reliability in comparisonwith the method for determining the D2D reception resource for eachterminal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system to which the presentinvention is applied.

FIG. 2 is a diagram showing a wireless protocol architecture for a userplane.

FIG. 3 is a diagram showing a wireless protocol architecture for acontrol plane.

FIG. 4 is a flowchart illustrating the operation of UE in the RRC idlestate.

FIG. 5 is a flowchart illustrating a process of establishing RRCconnection.

FIG. 6 is a flowchart illustrating an RRC connection reconfigurationprocess.

FIG. 7 is a diagram illustrating an RRC connection re-establishmentprocedure.

FIG. 8 illustrates substrates which may be owned by UE in the RRC_IDLEstate and a substrate transition process.

FIG. 9 shows a basic structure for ProSe.

FIG. 10 shows the deployment examples of types of UE performing ProSedirect communication and cell coverage.

FIG. 11 shows a user plane protocol stack for ProSe directcommunication.

FIG. 12 shows the PC 5 interface for D2D direct discovery.

FIG. 13 is an embodiment of a ProSe discovery process.

FIG. 14 is another embodiment of a ProSe discovery process.

FIG. 15 illustrates a D2D operation method performed by a UE accordingto an embodiment of the present invention.

FIG. 16 exemplifies a D2D operation method of a UE.

FIG. 17 is a block diagram illustrating a UE in which the embodiments ofthe present invention are implemented.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 shows a wireless communication system to which the presentinvention is applied. The wireless communication system may also bereferred to as an evolved-UMTS terrestrial radio access network(E-UTRAN) or a long term evolution (LTE)/LTE-A system.

The E-UTRAN includes at least one base station (BS) 20 which provides acontrol plane and a user plane to a user equipment (UE) 10. The UE 10may be fixed or mobile, and may be referred to as another terminology,such as a mobile station (MS), a user terminal (UT), a subscriberstation (SS), a mobile terminal (MT), a wireless device, etc. The BS 20is generally a fixed station that communicates with the UE 10 and may bereferred to as another terminology, such as an evolved node-B (eNB), abase transceiver system (BTS), an access point, etc.

The BSs 20 are interconnected by means of an X2 interface. The BSs 20are also connected by means of an S1 interface to an evolved packet core(EPC) 30, more specifically, to a mobility management entity (MME)through S1-MME and to a serving gateway (S-GW) through S1-U.

The EPC 30 includes an MME, an S-GW, and a packet data network-gateway(P-GW). The MME has access information of the UE or capabilityinformation of the UE, and such information is generally used formobility management of the UE. The S-GW is a gateway having an E-UTRANas an end point. The P-GW is a gateway having a PDN as an end point.

Layers of a radio interface protocol between the UE and the network canbe classified into a first layer (L1), a second layer (L2), and a thirdlayer (L3) based on the lower three layers of the open systeminterconnection (OSI) model that is well-known in the communicationsystem. Among them, a physical (PHY) layer belonging to the first layerprovides an information transfer service by using a physical channel,and a radio resource control (RRC) layer belonging to the third layerserves to control a radio resource between the UE and the network. Forthis, the RRC layer exchanges an RRC message between the UE and the BS.

FIG. 2 is a diagram showing a wireless protocol architecture for a userplane. FIG. 3 is a diagram showing a wireless protocol architecture fora control plane. The user plane is a protocol stack for user datatransmission. The control plane is a protocol stack for control signaltransmission.

Referring to FIGS. 2 and 3, a PHY layer provides an upper layer with aninformation transfer service through a physical channel. The PHY layeris connected to a medium access control (MAC) layer which is an upperlayer of the PHY layer through a transport channel Data is transferredbetween the MAC layer and the PHY layer through the transport channel.The transport channel is classified according to how and with whatcharacteristics data is transferred through a radio interface.

Data is moved between different PHY layers, that is, the PHY layers of atransmitter and a receiver, through a physical channel. The physicalchannel may be modulated according to an Orthogonal Frequency DivisionMultiplexing (OFDM) scheme, and use the time and frequency as radioresources.

The functions of the MAC layer include mapping between a logical channeland a transport channel and multiplexing and demultiplexing to atransport block that is provided through a physical channel on thetransport channel of a MAC Service Data Unit (SDU) that belongs to alogical channel. The MAC layer provides service to a Radio Link Control(RLC) layer through the logical channel.

The functions of the RLC layer include the concatenation, segmentation,and reassembly of an RLC SDU. In order to guarantee various types ofQuality of Service (QoS) required by a Radio Bearer (RB), the RLC layerprovides three types of operation mode: Transparent Mode (TM),Unacknowledged Mode (UM), and Acknowledged Mode (AM). AM RLC provideserror correction through an Automatic Repeat Request (ARQ).

The RRC layer is defined only on the control plane. The RRC layer isrelated to the configuration, reconfiguration, and release of radiobearers, and is responsible for control of logical channels, transportchannels, and PHY channels. An RB means a logical route that is providedby the first layer (PHY layer) and the second layers (MAC layer, the RLClayer, and the PDCP layer) in order to transfer data between UE and anetwork.

The function of a Packet Data Convergence Protocol (PDCP) layer on theuser plane includes the transfer of user data and header compression andciphering. The function of the PDCP layer on the user plane furtherincludes the transfer and encryption/integrity protection of controlplane data.

What an RB is configured means a process of defining the characteristicsof a wireless protocol layer and channels in order to provide specificservice and configuring each detailed parameter and operating method. AnRB can be divided into two types of a Signaling RB (SRB) and a Data RB(DRB). The SRB is used as a passage through which an RRC message istransmitted on the control plane, and the DRB is used as a passagethrough which user data is transmitted on the user plane.

If RRC connection is established between the RRC layer of UE and the RRClayer of an E-UTRAN, the UE is in the RRC connected state. If not, theUE is in the RRC idle state.

A downlink transport channel through which data is transmitted from anetwork to UE includes a broadcast channel (BCH) through which systeminformation is transmitted and a downlink shared channel (SCH) throughwhich user traffic or control messages are transmitted. Traffic or acontrol message for downlink multicast or broadcast service may betransmitted through the downlink SCH, or may be transmitted through anadditional downlink multicast channel (MCH). Meanwhile, an uplinktransport channel through which data is transmitted from UE to a networkincludes a random access channel (RACH) through which an initial controlmessage is transmitted and an uplink shared channel (SCH) through whichuser traffic or control messages are transmitted.

Logical channels that are placed over the transport channel and that aremapped to the transport channel include a broadcast control channel(BCCH), a paging control channel (PCCH), a common control channel(CCCH), a multicast control channel (MCCH), and a multicast trafficchannel (MTCH).

The physical channel includes several OFDM symbols in the time domainand several subcarriers in the frequency domain One subframe includes aplurality of OFDM symbols in the time domain. An RB is a resourcesallocation unit, and includes a plurality of OFDM symbols and aplurality of subcarriers. Furthermore, each subframe may use specificsubcarriers of specific OFDM symbols (e.g., the first OFDM symbol) ofthe corresponding subframe for a physical downlink control channel(PDCCH), that is, an L1/L2 control channel. A Transmission Time Interval(TTI) is a unit time for subframe transmission.

The RRC state of UE and an RRC connection method are described below.

The RRC state means whether or not the RRC layer of UE is logicallyconnected to the RRC layer of the E-UTRAN. A case where the RRC layer ofUE is logically connected to the RRC layer of the E-UTRAN is referred toas an RRC connected state. A case where the RRC layer of UE is notlogically connected to the RRC layer of the E-UTRAN is referred to as anRRC idle state. The E-UTRAN may check the existence of corresponding UEin the RRC connected state in each cell because the UE has RRCconnection, so the UE may be effectively controlled. In contrast, theE-UTRAN is unable to check UE in the RRC idle state, and a Core Network(CN) manages UE in the RRC idle state in each tracking area, that is,the unit of an area greater than a cell. That is, the existence ornon-existence of UE in the RRC idle state is checked only for each largearea. Accordingly, the UE needs to shift to the RRC connected state inorder to be provided with common mobile communication service, such asvoice or data.

When a user first powers UE, the UE first searches for a proper cell andremains in the RRC idle state in the corresponding cell. The UE in theRRC idle state establishes RRC connection with an E-UTRAN through an RRCconnection procedure when it is necessary to set up the RRC connection,and shifts to the RRC connected state. A case where UE in the RRC idlestate needs to set up RRC connection includes several cases. Forexample, the cases may include a need to send uplink data for a reason,such as a call attempt by a user, and to send a response message as aresponse to a paging message received from an E-UTRAN.

A Non-Access Stratum (NAS) layer placed over the RRC layer performsfunctions, such as session management and mobility management.

In the NAS layer, in order to manage the mobility of UE, two types ofstates: EPS Mobility Management-REGISTERED (EMM-REGISTERED) andEMM-DEREGISTERED are defined. The two states are applied to UE and theMME. UE is initially in the EMM-DEREGISTERED state. In order to access anetwork, the UE performs a process of registering it with thecorresponding network through an initial attach procedure. If the attachprocedure is successfully performed, the UE and the MME become theEMM-REGISTERED state.

In order to manage signaling connection between UE and the EPC, twotypes of states: an EPS Connection Management (ECM)-IDLE state and anECM-CONNECTED state are defined. The two states are applied to UE andthe MME. When the UE in the ECM-IDLE state establishes RRC connectionwith the E-UTRAN, the UE becomes the ECM-CONNECTED state. The MME in theECM-IDLE state becomes the ECM-CONNECTED state when it establishes S1connection with the E-UTRAN. When the UE is in the ECM-IDLE state, theE-UTRAN does not have information about the context of the UE.Accordingly, the UE in the ECM-IDLE state performs procedures related toUE-based mobility, such as cell selection or cell reselection, without aneed to receive a command from a network. In contrast, when the UE is inthe ECM-CONNECTED state, the mobility of the UE is managed in responseto a command from a network. If the location of the UE in the ECM-IDLEstate is different from a location known to the network, the UE informsthe network of its corresponding location through a tracking area updateprocedure.

System information is described below.

System information includes essential information that needs to be knownby UE in order for the UE to access a BS. Accordingly, the UE needs tohave received all pieces of system information before accessing the BS,and needs to always have the up-to-date system information. Furthermore,the BS periodically transmits the system information because the systeminformation is information that needs to be known by all UEs within onecell. The system information is divided into a Master Information Block(MIB) and a plurality of System Information Blocks (SIBs).

The MIB may include the limited number of parameters which are the mostessential and are most frequently transmitted in order to obtain otherinformation from a cell. UE first discovers an MIB after downlinksynchronization. The MIB may include information, such as a downlinkchannel bandwidth, a PHICH configuration, an SFN supportingsynchronization and operating as a timing reference, and an eNBtransmission antenna configuration. The MIB may be broadcasted on a BCH.

SystemInformationBlockType1 (SIB1) of included SIBs is included in a“SystemInformationBlockType1” message and transmitted. Other SIBs otherthan the SIB1 are included in a system information message andtransmitted. The mapping of the SIBs to the system information messagemay be flexibly configured by a scheduling information list parameterincluded in the SIB1. In this case, each SIB is included in a singlesystem information message. Only SIBs having the same schedulingrequired value (e.g. period) may be mapped to the same systeminformation message. Furthermore, SystemInformationBlockType2 (SIB2) isalways mapped to a system information message corresponding to the firstentry within the system information message list of a schedulinginformation list. A plurality of system information messages may betransmitted within the same period. The SIB1 and all of the systeminformation messages are transmitted on a DL-SCH.

In addition to broadcast transmission, in the E-UTRAN, the SIB1 may bechannel-dedicated signaling including a parameter set to have the samevalue as an existing set value. In this case, the SIB1 may be includedin an RRC connection re-establishment message and transmitted.

The SIB1 includes information related to UE cell access and defines thescheduling of other SIBs. The SIB1 may include information related tothe PLMN identifiers, Tracking Area Code (TAC), and cell ID of anetwork, a cell barring state indicative of whether a cell is a cell onwhich UE can camp, a required minimum reception level within a cellwhich is used as a cell reselection reference, and the transmission timeand period of other SIBs.

The SIB2 may include radio resource configuration information common toall types of UE. The SIB2 may include information related to an uplinkcarrier frequency and uplink channel bandwidth, an RACH configuration, apage configuration, an uplink power control configuration, a soundingreference signal configuration, a PUCCH configuration supportingACK/NACK transmission, and a PUSCH configuration.

UE may apply a procedure for obtaining system information and fordetecting a change of system information to only a PCell. In an SCell,when the corresponding SCell is added, the E-UTRAN may provide all typesof system information related to an RRC connection state operationthrough dedicated signaling. When system information related to aconfigured SCell is changed, the E-UTRAN may release a considered SCelland add the considered SCell later. This may be performed along with asingle RRC connection re-establishment message. The E-UTRAN may set avalue broadcast within a considered SCell and other parameter valuethrough dedicated signaling.

UE needs to guarantee the validity of a specific type of systeminformation. Such system information is called required systeminformation. The required system information may be defined as follows.

-   -   If UE is in the RRC_IDLE state: the UE needs to have the valid        version of the MIB and the SIB1 in addition to the SIB2 to SIB8.        This may comply with the support of a considered RAT.    -   If UE is in the RRC connection state: the UE needs to have the        valid version of the MIB, SIB1, and SIB2.

In general, the validity of system information may be guaranteed up to amaximum of 3 hours after being obtained.

In general, service that is provided to UE by a network may beclassified into three types as follows. Furthermore, the UE differentlyrecognizes the type of cell depending on what service may be provided tothe UE. In the following description, a service type is first described,and the type of cell is described.

1) Limited service: this service provides emergency calls and anEarthquake and Tsunami Warning System (ETWS), and may be provided by anacceptable cell.

2) Suitable service: this service means public service for common uses,and may be provided by a suitable cell (or a normal cell).

3) Operator service: this service means service for communicationnetwork operators. This cell may be used by only communication networkoperators, but may not be used by common users.

In relation to a service type provided by a cell, the type of cell maybe classified as follows.

1) An acceptable cell: this cell is a cell from which UE may be providedwith limited service. This cell is a cell that has not been barred froma viewpoint of corresponding UE and that satisfies the cell selectioncriterion of the UE.

2) A suitable cell: this cell is a cell from which UE may be providedwith suitable service. This cell satisfies the conditions of anacceptable cell and also satisfies additional conditions. The additionalconditions include that the suitable cell needs to belong to a PublicLand Mobile Network (PLMN) to which corresponding UE may access and thatthe suitable cell is a cell on which the execution of a tracking areaupdate procedure by the UE is not barred. If a corresponding cell is aCSG cell, the cell needs to be a cell to which UE may access as a memberof the CSG.

3) A barred cell: this cell is a cell that broadcasts informationindicative of a barred cell through system information.

4) A reserved cell: this cell is a cell that broadcasts informationindicative of a reserved cell through system information.

FIG. 4 is a flowchart illustrating the operation of UE in the RRC idlestate. FIG. 4 illustrates a procedure in which UE that is initiallypowered on experiences a cell selection process, registers it with anetwork, and then performs cell reselection if necessary.

Referring to FIG. 4, the UE selects Radio Access Technology (RAT) inwhich the UE communicates with a Public Land Mobile Network (PLMN), thatis, a network from which the UE is provided with service (S410).Information about the PLMN and the RAT may be selected by the user ofthe UE, and the information stored in a Universal Subscriber IdentityModule (USIM) may be used.

The UE selects a cell that has the greatest value and that belongs tocells having measured BS and signal intensity or quality greater than aspecific value (cell selection) (S420). In this case, the UE that ispowered off performs cell selection, which may be called initial cellselection. A cell selection procedure is described later in detail.After the cell selection, the UE receives system informationperiodically by the BS. The specific value refers to a value that isdefined in a system in order for the quality of a physical signal indata transmission/reception to be guaranteed. Accordingly, the specificvalue may differ depending on applied RAT.

If network registration is necessary, the UE performs a networkregistration procedure (S430). The UE registers its information (e.g.,an IMSI) with the network in order to receive service (e.g., paging)from the network. The UE does not register it with a network whenever itselects a cell, but registers it with a network when information aboutthe network (e.g., a Tracking Area Identity (TAI)) included in systeminformation is different from information about the network that isknown to the UE.

The UE performs cell reselection based on a service environment providedby the cell or the environment of the UE (S440). If the value of theintensity or quality of a signal measured based on a BS from which theUE is provided with service is lower than that measured based on a BS ofa neighboring cell, the UE selects a cell that belongs to other cellsand that provides better signal characteristics than the cell of the BSthat is accessed by the UE. This process is called cell reselectiondifferently from the initial cell selection of the No. 2 process. Inthis case, temporal restriction conditions are placed in order for acell to be frequently reselected in response to a change of signalcharacteristic. A cell reselection procedure is described later indetail.

FIG. 5 is a flowchart illustrating a process of establishing RRCconnection.

UE sends an RRC connection request message that requests RRC connectionto a network (S510). The network sends an RRC connection establishmentmessage as a response to the RRC connection request (S520). Afterreceiving the RRC connection establishment message, the UE enters RRCconnected mode.

The UE sends an RRC connection establishment complete message used tocheck the successful completion of the RRC connection to the network(S530).

FIG. 6 is a flowchart illustrating an RRC connection reconfigurationprocess. An RRC connection reconfiguration is used to modify RRCconnection. This is used to establish/modify/release RBs, performhandover, and set up/modify/release measurements.

A network sends an RRC connection reconfiguration message for modifyingRRC connection to UE (S610). As a response to the RRC connectionreconfiguration message, the UE sends an RRC connection reconfigurationcomplete message used to check the successful completion of the RRCconnection reconfiguration to the network (S620).

Hereinafter, a public land mobile network (PLMN) is described.

The PLMN is a network which is disposed and operated by a mobile networkoperator. Each mobile network operator operates one or more PLMNs. EachPLMN may be identified by a Mobile Country Code (MCC) and a MobileNetwork Code (MNC). PLMN information of a cell is included in systeminformation and broadcasted.

In PLMN selection, cell selection, and cell reselection, various typesof PLMNs may be considered by the terminal.

Home PLMN (HPLMN): PLMN having MCC and MNC matching with MCC and MNC ofa terminal IMSI.

Equivalent HPLMN (EHPLMN): PLMN serving as an equivalent of an HPLMN.

Registered PLMN (RPLMN): PLMN successfully finishing locationregistration.

Equivalent PLMN (EPLMN): PLMN serving as an equivalent of an RPLMN.

Each mobile service consumer subscribes in the HPLMN. When a generalservice is provided to the terminal through the HPLMN or the EHPLMN, theterminal is not in a roaming state. Meanwhile, when the service isprovided to the terminal through a PLMN except for the HPLMN/EHPLMN, theterminal is in the roaming state. In this case, the PLMN refers to aVisited PLMN (VPLMN).

When UE is initially powered on, the UE searches for available PublicLand Mobile Networks (PLMNs) and selects a proper PLMN from which the UEis able to be provided with service. The PLMN is a network that isdeployed or operated by a mobile network operator. Each mobile networkoperator operates one or more PLMNs. Each PLMN may be identified byMobile Country Code (MCC) and Mobile Network Code (MNC). Informationabout the PLMN of a cell is included in system information andbroadcasted. The UE attempts to register it with the selected PLMN. Ifregistration is successful, the selected PLMN becomes a Registered PLMN(RPLMN). The network may signalize a PLMN list to the UE. In this case,PLMNs included in the PLMN list may be considered to be PLMNs, such asRPLMNs. The UE registered with the network needs to be able to be alwaysreachable by the network. If the UE is in the ECM-CONNECTED state(identically the RRC connection state), the network recognizes that theUE is being provided with service. If the UE is in the ECM-IDLE state(identically the RRC idle state), however, the situation of the UE isnot valid in an eNB, but is stored in the MME. In such a case, only theMME is informed of the location of the UE in the ECM-IDLE state throughthe granularity of the list of Tracking Areas (TAs). A single TA isidentified by a Tracking Area Identity (TAI) formed of the identifier ofa PLMN to which the TA belongs and Tracking Area Code (TAC) thatuniquely expresses the TA within the PLMN.

Thereafter, the UE selects a cell that belongs to cells provided by theselected PLMN and that has signal quality and characteristics on whichthe UE is able to be provided with proper service.

The following is a detailed description of a procedure of selecting acell by a terminal.

When power is turned-on or the terminal is located in a cell, theterminal performs procedures for receiving a service byselecting/reselecting a suitable quality cell.

A terminal in an RRC idle state should prepare to receive a servicethrough the cell by always selecting a suitable quality cell. Forexample, a terminal where power is turned-on just before should selectthe suitable quality cell to be registered in a network. If the terminalin an RRC connection state enters in an RRC idle state, the terminalshould selects a cell for stay in the RRC idle state. In this way, aprocedure of selecting a cell satisfying a certain condition by theterminal in order to be in a service idle state such as the RRC idlestate refers to cell selection. Since the cell selection is performed ina state that a cell in the RRC idle state is not currently determined,it is important to select the cell as rapid as possible. Accordingly, ifthe cell provides a wireless signal quality of a predetermined level orgreater, although the cell does not provide the best wireless signalquality, the cell may be selected during a cell selection procedure ofthe terminal.

A method and a procedure of selecting a cell by a terminal in a 3GPP LTEis described with reference to 3GPP TS 36.304 V8.5.0 (2009 March) “UserEquipment (UE) procedures in idle mode (Release 8)”.

A cell selection process is basically divided into two types.

The first is an initial cell selection process. In this process, UE doesnot have preliminary information about a wireless channel. Accordingly,the UE searches for all wireless channels in order to find out a propercell. The UE searches for the strongest cell in each channel.Thereafter, if the UE has only to search for a suitable cell thatsatisfies a cell selection criterion, the UE selects the correspondingcell.

Next, the UE may select the cell using stored information or usinginformation broadcasted by the cell. Accordingly, cell selection may befast compared to an initial cell selection process. If the UE has onlyto search for a cell that satisfies the cell selection criterion, the UEselects the corresponding cell. If a suitable cell that satisfies thecell selection criterion is not retrieved though such a process, the UEperforms an initial cell selection process.

The cell selection criterion may be defined as below equation 1.Srxlev>0 AND Squal>0where:Srxlev=Q _(rxlevmeas)−(Q _(rxlev min) +Q_(rxlev min offset))−PcompensationSqual=Q _(qualmeas)−(Q _(qual min) +Q _(qual min offset))  [Equation 1]

Here, the variables in the equation 1 may be defined as below table 1.

TABLE 1 Srxlev Cell selection RX level value (dB) Squal Cell selectionquality value (dB) Q_(rxlevmeas) Measured cell RX level value (RSRP)Q_(qualmeas) Measured cell quality value (RSRQ) Q_(rxlevmin) Minimumrequired RX level in the cell (dBm) Q_(qualmin) Minimum required qualitylevel in the cell (dB) Q_(rxlevminoffset) Offset to the signalledQ_(rxlevmin) taken into account in the Srxlev evaluation as a result ofa periodic search for a higher priority PLMN while camped normally in aVPLMN Q_(qualminoffset) Offset to the signalled Q_(qualmin) taken intoaccount in the Squal evaluation as a result of a periodic search for ahigher priority PLMN while camped normally in a VPLMN Pcompensationmax(P_(EMAX) − P_(PowerClass), 0) (dB) P_(EMAX) Maximum TX power levelan UE may use when transmitting on the uplink in the cell (dBm) definedas P_(EMAX) in [TS 36.101] P_(PowerClass) Maximum RF output power of theUE (dBm) according to the UE power class as defined in [TS 36.101]

Signalled values, i.e., Q_(rxlev min offset) and Q_(qual min offset),may be applied to a case where cell selection is evaluated as a resultof periodic search for a higher priority PLMN during a UE camps on anormal cell in a VPLMN. During the periodic search for the higherpriority PLMN as described above, the UE may perform the cell selectionevaluation by using parameter values stored in other cells of the higherpriority PLMN.

After the UE selects a specific cell through the cell selection process,the intensity or quality of a signal between the UE and a BS may bechanged due to a change in the mobility or wireless environment of theUE. Accordingly, if the quality of the selected cell is deteriorated,the UE may select another cell that provides better quality. If a cellis reselected as described above, the UE selects a cell that providesbetter signal quality than the currently selected cell. Such a processis called cell reselection. In general, a basic object of the cellreselection process is to select a cell that provides UE with the bestquality from a viewpoint of the quality of a radio signal.

In addition to the viewpoint of the quality of a radio signal, a networkmay determine priority corresponding to each frequency, and may informthe UE of the determined priorities. The UE that has received thepriorities preferentially takes into consideration the priorities in acell reselection process compared to a radio signal quality criterion.

As described above, there is a method of selecting or reselecting a cellaccording to the signal characteristics of a wireless environment. Inselecting a cell for reselection when a cell is reselected, thefollowing cell reselection methods may be present according to the RATand frequency characteristics of the cell.

-   -   Intra-frequency cell reselection: UE reselects a cell having the        same center frequency as that of RAT, such as a cell on which        the UE camps on.    -   Inter-frequency cell reselection: UE reselects a cell having a        different center frequency from that of RAT, such as a cell on        which the UE camps on    -   Inter-RAT cell reselection: UE reselects a cell that uses RAT        different from RAT on which the UE camps

The principle of a cell reselection process is as follows.

First, UE measures the quality of a serving cell and neighbor cells forcell reselection.

Second, cell reselection is performed based on a cell reselectioncriterion. The cell reselection criterion has the followingcharacteristics in relation to the measurements of a serving cell andneighbor cells.

Intra-frequency cell reselection is basically based on ranking. Rankingis a task for defining a criterion value for evaluating cell reselectionand numbering cells using criterion values according to the size of thecriterion values. A cell having the best criterion is commonly calledthe best-ranked cell. The cell criterion value is based on the value ofa corresponding cell measured by UE, and may be a value to which afrequency offset or cell offset has been applied, if necessary.

Inter-frequency cell reselection is based on frequency priority providedby a network. UE attempts to camp on a frequency having the highestfrequency priority. A network may provide frequency priority that willbe applied by UEs within a cell in common through broadcastingsignaling, or may provide frequency-specific priority to each UE throughUE-dedicated signaling. A cell reselection priority provided throughbroadcast signaling may refer to a common priority. A cell reselectionpriority for each terminal set by a network may refer to a dedicatedpriority. If receiving the dedicated priority, the terminal may receivea valid time associated with the dedicated priority together. Ifreceiving the dedicated priority, the terminal starts a validity timerset as the received valid time together therewith. While the valid timeris operated, the terminal applies the dedicated priority in the RRC idlemode. If the valid timer is expired, the terminal discards the dedicatedpriority and again applies the common priority.

For the inter-frequency cell reselection, a network may provide UE witha parameter (e.g., a frequency-specific offset) used in cell reselectionfor each frequency.

For the intra-frequency cell reselection or the inter-frequency cellreselection, a network may provide UE with a Neighboring Cell List (NCL)used in cell reselection. The NCL includes a cell-specific parameter(e.g., a cell-specific offset) used in cell reselection.

For the intra-frequency or inter-frequency cell reselection, a networkmay provide UE with a cell reselection black list used in cellreselection. The UE does not perform cell reselection on a cell includedin the black list.

Ranking performed in a cell reselection evaluation process is describedbelow.

A ranking criterion used to apply priority to a cell is defined as inEquation 1.Rs=Qmeas,s+Qhyst, Rn=Qmeas,s−Qoffset  [Equation 2]

In this case, Rs is the ranking criterion of a serving cell, Rn is theranking criterion of a neighbor cell, Qmeas,s is the quality value ofthe serving cell measured by UE, Qmeas,n is the quality value of theneighbor cell measured by UE, Qhyst is the hysteresis value for ranking,and Qoffset is an offset between the two cells.

In Intra-frequency, if UE receives an offset “Qoffsets,n” between aserving cell and a neighbor cell, Qoffset=Qoffsets,n. If UE does notQoffsets,n, Qoffset=0.

In Inter-frequency, if UE receives an offset “Qoffsets,n” for acorresponding cell, Qoffset=Qoffsets,n+Qfrequency. If UE does notreceive “Qoffsets,n”, Qoffset=Qfrequency.

If the ranking criterion Rs of a serving cell and the ranking criterionRn of a neighbor cell are changed in a similar state, ranking priorityis frequency changed as a result of the change, and UE may alternatelyreselect the twos. Qhyst is a parameter that gives hysteresis to cellreselection so that UE is prevented from to alternately reselecting twocells.

UE measures RS of a serving cell and Rn of a neighbor cell according tothe above equation, considers a cell having the greatest rankingcriterion value to be the best-ranked cell, and reselects the cell.

In accordance with the criterion, it may be checked that the quality ofa cell is the most important criterion in cell reselection. If areselected cell is not a suitable cell, UE excludes a correspondingfrequency or a corresponding cell from the subject of cell res election.

A Radio Link Failure (RLF) is described below.

UE continues to perform measurements in order to maintain the quality ofa radio link with a serving cell from which the UE receives service. TheUE determines whether or not communication is impossible in a currentsituation due to the deterioration of the quality of the radio link withthe serving cell. If communication is almost impossible because thequality of the serving cell is too low, the UE determines the currentsituation to be an RLE

If the RLF is determined, the UE abandons maintaining communication withthe current serving cell, selects a new cell through cell selection (orcell reselection) procedure, and attempts RRC connectionre-establishment with the new cell.

In the specification of 3GPP LTE, the following examples are taken ascases where normal communication is impossible.

-   -   A case where UE determines that there is a serious problem in        the quality of a downlink communication link (a case where the        quality of a PCell is determined to be low while performing RLM)        based on the radio quality measured results of the PHY layer of        the UE    -   A case where uplink transmission is problematic because a random        access procedure continues to fail in the MAC sublayer.    -   A case where uplink transmission is problematic because uplink        data transmission continues to fail in the RLC sublayer.    -   A case where handover is determined to have failed.    -   A case where a message received by UE does not pass through an        integrity check.

An RRC connection re-establishment procedure is described in more detailbelow.

FIG. 7 is a diagram illustrating an RRC connection re-establishmentprocedure.

Referring to FIG. 7, UE stops using all the radio bearers that have beenconfigured other than a Signaling Radio Bearer (SRB) #0, and initializesa variety of kinds of sublayers of an Access Stratum (AS) (S710).Furthermore, the UE configures each sublayer and the PHY layer as adefault configuration. In this process, the UE maintains the RRCconnection state.

The UE performs a cell selection procedure for performing an RRCconnection reconfiguration procedure (S720). The cell selectionprocedure of the RRC connection re-establishment procedure may beperformed in the same manner as the cell selection procedure that isperformed by the UE in the RRC idle state, although the UE maintains theRRC connection state.

After performing the cell selection procedure, the UE determines whetheror not a corresponding cell is a suitable cell by checking the systeminformation of the corresponding cell (S730). If the selected cell isdetermined to be a suitable E-UTRAN cell, the UE sends an RRC connectionre-establishment request message to the corresponding cell (S740).

Meanwhile, if the selected cell is determined to be a cell that uses RATdifferent from that of the E-UTRAN through the cell selection procedurefor performing the RRC connection re-establishment procedure, the UEstops the RRC connection re-establishment procedure and enters the RRCidle state (S750).

The UE may be implemented to finish checking whether the selected cellis a suitable cell through the cell selection procedure and thereception of the system information of the selected cell. To this end,the UE may drive a timer when the RRC connection re-establishmentprocedure is started. The timer may be stopped if it is determined thatthe UE has selected a suitable cell. If the timer expires, the UE mayconsider that the RRC connection re-establishment procedure has failed,and may enter the RRC idle state. Such a timer is hereinafter called anRLF timer. In LTE spec TS 36.331, a timer named “T311” may be used as anRLF timer. The UE may obtain the set value of the timer from the systeminformation of the serving cell.

If an RRC connection re-establishment request message is received fromthe UE and the request is accepted, a cell sends an RRC connectionre-establishment message to the UE.

The UE that has received the RRC connection re-establishment messagefrom the cell reconfigures a PDCP sublayer and an RLC sublayer with anSRB1. Furthermore, the UE calculates various key values related tosecurity setting, and reconfigures a PDCP sublayer responsible forsecurity as the newly calculated security key values. Accordingly, theSRB1 between the UE and the cell is open, and the UE and the cell mayexchange RRC control messages. The UE completes the restart of the SRB1,and sends an RRC connection re-establishment complete message indicativeof that the RRC connection re-establishment procedure has been completedto the cell (S760).

In contrast, if the RRC connection re-establishment request message isreceived from the UE and the request is not accepted, the cell sends anRRC connection re-establishment reject message to the UE.

If the RRC connection re-establishment procedure is successfullyperformed, the cell and the UE perform an RRC connection reconfigurationprocedure. Accordingly, the UE recovers the state prior to the executionof the RRC connection re-establishment procedure, and the continuity ofservice is guaranteed to the upmost.

FIG. 8 illustrates substrates which may be owned by UE in the RRC_IDLEstate and a substrate transition process.

Referring to FIG. 8, UE performs an initial cell selection process(S801). The initial cell selection process may be performed when thereis no cell information stored with respect to a PLMN or if a suitablecell is not discovered.

If a suitable cell is unable to be discovered in the initial cellselection process, the UE transits to any cell selection state (S802).The any cell selection state is the state in which the UE has not campedon a suitable cell and an acceptable cell and is the state in which theUE attempts to discover an acceptable cell of a specific PLMN on whichthe UE may camp. If the UE has not discovered any cell on which it maycamp, the UE continues to stay in the any cell selection state until itdiscovers an acceptable cell.

If a suitable cell is discovered in the initial cell selection process,the UE transits to a normal camp state (S803). The normal camp staterefers to the state in which the UE has camped on the suitable cell. Inthis state, the UE may select and monitor a paging channel based oninformation provided through system information and may perform anevaluation process for cell reselection.

If a cell reselection evaluation process (S804) is caused in the normalcamp state (S803), the UE performs a cell reselection evaluation process(S804). If a suitable cell is discovered in the cell reselectionevaluation process (S804), the UE transits to the normal camp state(S803) again.

If an acceptable cell is discovered in the any cell selection state(S802), the UE transmits to any cell camp state (S805). The any cellcamp state is the state in which the UE has camped on the acceptablecell.

In the any cell camp state (S805), the UE may select and monitor apaging channel based on information provided through system informationand may perform the evaluation process (S806) for cell reselection. Ifan acceptable cell is not discovered in the evaluation process (S806)for cell reselection, the UE transits to the any cell selection state(S802).

Now, a device-to-device (D2D) operation is described. In 3GPP LTE-A, aservice related to the D2D operation is called a proximity service(ProSe). Now, the ProSe is described. Hereinafter, the ProSe is the sameconcept as the D2D operation, and the ProSe and the D2D operation may beused without distinction.

The ProSe includes ProSe direction communication and ProSe directdiscovery. The ProSe direct communication is communication performedbetween two or more proximate UEs. The UEs may perform communication byusing a protocol of a user plane. A ProSe-enabled UE implies a UEsupporting a procedure related to a requirement of the ProSe. Unlessotherwise specified, the ProSe-enabled UE includes both of a publicsafety UE and a non-public safety UE. The public safety UE is a UEsupporting both of a function specified for a public safety and a ProSeprocedure, and the non-public safety UE is a UE supporting the ProSeprocedure and not supporting the function specified for the publicsafety.

ProSe direct discovery is a process for discovering anotherProSe-enabled UE adjacent to ProSe-enabled UE. In this case, only thecapabilities of the two types of ProSe-enabled UE are used. EPC-levelProSe discovery means a process for determining, by an EPC, whether thetwo types of ProSe-enabled UE are in proximity and notifying the twotypes of ProSe-enabled UE of the proximity.

Hereinafter, for convenience, the ProSe direct communication may bereferred to as D2D communication, and the ProSe direct discovery may bereferred to as D2D discovery.

FIG. 9 shows a basic structure for ProSe.

Referring to FIG. 9, the basic structure for ProSe includes an E-UTRAN,an EPC, a plurality of types of UE including a ProSe applicationprogram, a ProSe application server (a ProSe APP server), and a ProSefunction.

The EPC represents an E-UTRAN core network configuration. The EPC mayinclude an MME, an S-GW, a P-GW, a policy and charging rules function(PCRF), a home subscriber server (HSS) and so on.

The ProSe APP server is a user of a ProSe capability for producing anapplication function. The ProSe APP server may communicate with anapplication program within UE. The application program within UE may usea ProSe capability for producing an application function.

The ProSe function may include at least one of the followings, but isnot necessarily limited thereto.

-   -   Interworking via a reference point toward the 3rd party        applications    -   Authorization and configuration of UE for discovery and direct        communication    -   Enable the functionality of EPC level ProSe discovery    -   ProSe related new subscriber data and handling of data storage,        and also handling of the ProSe identities    -   Security related functionality    -   Provide control towards the EPC for policy related functionality    -   Provide functionality for charging (via or outside of the EPC,        e.g., offline charging)

A reference point and a reference interface in the basic structure forProSe are described below.

-   -   PC1: a reference point between the ProSe application program        within the UE and the ProSe application program within the ProSe        APP server. This is used to define signaling requirements in an        application dimension.    -   PC2: a reference point between the ProSe APP server and the        ProSe function. This is used to define an interaction between        the ProSe APP server and the ProSe function. The update of        application data in the ProSe database of the ProSe function may        be an example of the interaction.    -   PC3: a reference point between the UE and the ProSe function.        This is used to define an interaction between the UE and the        ProSe function. A configuration for ProSe discovery and        communication may be an example of the interaction.    -   PC4: a reference point between the EPC and the ProSe function.        This is used to define an interaction between the EPC and the        ProSe function. The interaction may illustrate the time when a        path for 1:1 communication between types of UE is set up or the        time when ProSe service for real-time session management or        mobility management is authenticated.    -   PC5: a reference point used for using control/user plane for        discovery and communication, relay, and 1:1 communication        between types of UE.    -   PC6: a reference point for using a function, such as ProSe        discovery, between users belonging to different PLMNs.    -   SGi: this may be used to exchange application data and types of        application dimension control information.

<ProSe Direct Communication>

ProSe direct communication is communication mode in which two types ofpublic safety UE can perform direct communication through a PC 5interface. Such communication mode may be supported when UE is suppliedwith services within coverage of an E-UTRAN or when UE deviates fromcoverage of an E-UTRAN.

FIG. 10 shows the deployment examples of types of UE performing ProSedirect communication and cell coverage.

Referring to FIG. 10(a), types of UE A and B may be placed outside cellcoverage. Referring to FIG. 10(b), UE A may be placed within cellcoverage, and UE B may be placed outside cell coverage. Referring toFIG. 10(c), types of UE A and B may be placed within single cellcoverage. Referring to FIG. 10(d), UE A may be placed within coverage ofa first cell, and UE B may be placed within coverage of a second cell.

ProSe direct communication may be performed between types of UE placedat various positions as in FIG. 10.

Meanwhile, the following IDs may be used in ProSe direct communication.

A source layer-2 ID: this ID identifies the sender of a packet in the PC5 interface.

A destination layer-2 ID: this ID identifies the target of a packet inthe PC 5 interface.

An SA L1 ID: this ID is the ID of scheduling assignment (SA) in the PC 5interface.

FIG. 11 shows a user plane protocol stack for ProSe directcommunication.

Referring to FIG. 11, the PC 5 interface includes a PDCH, RLC, MAC, andPHY layers.

In ProSe direct communication, HARQ feedback may not be present. An MACheader may include a source layer-2 ID and a destination layer-2 ID.

<Radio Resource Assignment for ProSe Direct Communication>

ProSe-enabled UE may use the following two types of mode for resourceassignment for ProSe direct communication.

1. Mode 1

Mode 1 is mode in which resources for ProSe direct communication arescheduled by an eNB. UE needs to be in the RRC CONNECTED state in orderto send data in accordance with mode 1. The UE requests a transmissionresource from an eNB. The eNB performs scheduling assignment andschedules resources for sending data. The UE may send a schedulingrequest to the eNB and send a ProSe Buffer Status Report (BSR). The eNBhas data to be subjected to ProSe direct communication by the UE basedon the ProSe BSR and determines that a resource for transmission isrequired.

2. Mode 2

Mode 2 is mode in which UE directly selects a resource. UE directlyselects a resource for ProSe direct communication in a resource pool.The resource pool may be configured by a network or may have beenpreviously determined.

Meanwhile, if UE has a serving cell, that is, if the UE is in theRRC_CONNECTED state with an eNB or is placed in a specific cell in theRRC_IDLE state, the UE is considered to be placed within coverage of theeNB.

If UE is placed outside coverage, only mode 2 may be applied. If the UEis placed within the coverage, the UE may use mode 1 or mode 2 dependingon the configuration of an eNB.

If another exception condition is not present, only when an eNB performsa configuration, UE may change mode from mode 1 to mode 2 or from mode 2to mode 1.

<ProSe Direct Discovery>

ProSe direct discovery refers to a procedure that is used forProSe-enabled UE to discover another ProSe-enabled UE in proximity andis also called D2D direct discovery. In this case, E-UTRA radio signalsthrough the PC 5 interface may be used. Information used in ProSe directdiscovery is hereinafter called discovery information.

FIG. 12 shows the PC 5 interface for D2D direct discovery.

Referring to FIG. 12, the PC 5 interface includes an MAC layer, a PHYlayer, and a ProSe Protocol layer, that is, a higher layer. The higherlayer (the ProSe Protocol) handles the permission of the announcementand monitoring of discovery information. The contents of the discoveryinformation are transparent to an access stratum (AS). The ProSeProtocol transfers only valid discovery information to the AS forannouncement.

The MAC layer receives discovery information from the higher layer (theProSe Protocol). An IP layer is not used to send discovery information.The MAC layer determines a resource used to announce discoveryinformation received from the higher layer. The MAC layer produces anMAC protocol data unit (PDU) for carrying discovery information andsends the MAC PDU to the physical layer. An MAC header is not added.

In order to announce discovery information, there are two types ofresource assignment.

1. Type 1

The type 1 is a method for assigning a resource for announcing discoveryinformation in a UE-not-specific manner. An eNB provides a resource poolconfiguration for discovery information announcement to types of UE. Theconfiguration may be signaled through the SIB.

UE autonomously selects a resource from an indicated resource pool andannounces discovery information using the selected resource. The UE mayannounce the discovery information through a randomly selected resourceduring each discovery period.

2. Type 2

The type 2 is a method for assigning a resource for announcing discoveryinformation in a UE-specific manner. UE in the RRC_CONNECTED state mayrequest a resource for discovery signal announcement from an eNB throughan RRC signal. The eNB may announce a resource for discovery signalannouncement through an RRC signal. A resource for discovery signalmonitoring may be assigned within a resource pool configured for typesof UE.

An eNB 1) may announce a type 1 resource pool for discovery signalannouncement to UE in the RRC_IDLE state through the SIB. Types of UEwhose ProSe direct discovery has been permitted use the type 1 resourcepool for discovery information announcement in the RRC_IDLE state.Alternatively, the eNB 2) announces that the eNB supports ProSe directdiscovery through the SIB, but may not provide a resource for discoveryinformation announcement. In this case, UE needs to enter theRRC_CONNECTED state for discovery information announcement.

An eNB may configure that UE has to use a type 1 resource pool fordiscovery information announcement or has to use a type 2 resourcethrough an RRC signal in relation to UE in the RRC_CONNECTED state.

FIG. 13 is an embodiment of a ProSe discovery process.

Referring to FIG. 13, it is assumed that UE A and UE B haveProSe-enabled application programs managed therein and have beenconfigured to have a ‘friend’ relation between them in the applicationprograms, that is, a relationship in which D2D communication may bepermitted between them. Hereinafter, the UE B may be represented as a‘friend’ of the UE A. The application program may be, for example, asocial networking program. ‘3GPP Layers’ correspond to the functions ofan application program for using ProSe discovery service, which havebeen defined by 3GPP.

Direct discovery between the types of UE A and B may experience thefollowing process.

1. First, the UE A performs regular application layer communication withthe APP server. The communication is based on an Application ProgramInterface (API).

2. The ProSe-enabled application program of the UE A receives a list ofapplication layer IDs having a ‘friend’ relation. In general, theapplication layer ID may have a network access ID form. For example, theapplication layer ID of the UE A may have a form, such as“adam@example.com.”

3. The UE A requests private expressions code for the user of the UE Aand private representation code for a friend of the user.

4. The 3GPP layers send a representation code request to the ProSeserver.

5. The ProSe server maps the application layer IDs, provided by anoperator or a third party APP server, to the private representationcode. For example, an application layer ID, such as adam@example.com,may be mapped to private representation code, such as“GTER543$#2FSJ67DFSF.” Such mapping may be performed based on parameters(e.g., a mapping algorithm, a key value and so on) received from the APPserver of a network.

6. The ProSe server sends the types of derived representation code tothe 3GPP layers. The 3GPP layers announce the successful reception ofthe types of representation code for the requested application layer IDto the ProSe-enabled application program. Furthermore, the 3GPP layersgenerate a mapping table between the application layer ID and the typesof representation code.

7. The ProSe-enabled application program requests the 3GPP layers tostart a discovery procedure. That is, the ProSe-enabled applicationprogram requests the 3GPP layers to start discovery when one of provided‘friends’ is placed in proximity to the UE A and direct communication ispossible. The 3GPP layers announces the private representation code(i.e., in the above example, “GTER543$#2FSJ67DFSF”, that is, the privaterepresentation code of adam@example.com) of the UE A. This ishereinafter called ‘announcement’. Mapping between the application layerID of the corresponding application program and the privaterepresentation code may be known to only ‘friends’ which have previouslyreceived such a mapping relation, and the ‘friends’ may perform suchmapping.

8. It is assumed that the UE B operates the same ProSe-enabledapplication program as the UE A and has executed the aforementioned 3 to6 steps. The 3GPP layers placed in the UE B may execute ProSe discovery.

9. When the UE B receives the aforementioned ‘announce’ from the UE A,the UE B determines whether the private representation code included inthe ‘announce’ is known to the UE B and whether the privaterepresentation code is mapped to the application layer ID. As describedthe 8 step, since the UE B has also executed the 3 to 6 steps, it isaware of the private representation code, mapping between the privaterepresentation code and the application layer ID, and correspondingapplication program of the UE A. Accordingly, the UE B may discover theUE A from the ‘announce’ of the UE A. The 3GPP layers announce thatadam@example.com has been discovered to the ProSe-enabled applicationprogram within the UE B.

In FIG. 13, the discovery procedure has been described by taking intoconsideration all of the types of UE A and B, the ProSe server, the APPserver and so on. From the viewpoint of the operation between the typesof UE A and B, the UE A sends (this process may be called announcement)a signal called announcement, and the UE B receives the announce anddiscovers the UE A. That is, from the aspect that an operation thatbelongs to operations performed by types of UE and that is directlyrelated to another UE is only step, the discovery process of FIG. 13 mayalso be called a single step discovery procedure.

FIG. 14 is another embodiment of a ProSe discovery process.

In FIG. 14, types of UE 1 to 4 are assumed to types of UE included inspecific group communication system enablers (GCSE) group. It is assumedthat the UE 1 is a discoverer and the types of UE 2, 3, and 4 arediscoveree. UE 5 is UE not related to the discovery process.

The UE 1 and the UE 2-4 may perform a next operation in the discoveryprocess.

First, the UE 1 broadcasts a target discovery request message (may behereinafter abbreviated as a discovery request message or M1) in orderto discover whether specific UE included in the GCSE group is inproximity. The target discovery request message may include the uniqueapplication program group ID or layer-2 group ID of the specific GCSEgroup. Furthermore, the target discovery request message may include theunique ID, that is, application program private ID of the UE 1. Thetarget discovery request message may be received by the types of UE 2,3, 4, and 5.

The UE 5 sends no response message. In contrast, the types of UE 2, 3,and 4 included in the GCSE group send a target discovery responsemessage (may be hereinafter abbreviated as a discovery response messageor M2) as a response to the target discovery request message. The targetdiscovery response message may include the unique application programprivate ID of UE sending the message.

An operation between types of UE in the ProSe discovery processdescribed with reference to FIG. 14 is described below. The discoverer(the UE 1) sends a target discovery request message and receives atarget discovery response message, that is, a response to the targetdiscovery request message. Furthermore, when the discoveree (e.g., theUE 2) receives the target discovery request message, it sends a targetdiscovery response message, that is, a response to the target discoveryrequest message. Accordingly, each of the types of UE performs theoperation of the 2 step. In this aspect, the ProSe discovery process ofFIG. 14 may be called a 2-step discovery procedure.

In addition to the discovery procedure described in FIG. 14, if the UE 1(the discoverer) sends a discovery conform message (may be hereinafterabbreviated as an M3), that is, a response to the target discoveryresponse message, this may be called a 3-step discovery procedure.

Hereinafter, the present invention will be described.

A network may provide the information related to a D2D operationaccording to a cell. For example, the information may indicate theresources that a UE may use for receiving D2D signals. The informationmay be broadcasted.

The resource that a UE uses for transmitting and receiving data in theRRC connected state for the conventional cellular communication may bedetermined by a BS and notified to the UE through the UE dedicatedsignaling. That is, the UE in the RRC connected state receives theconfiguration of resource that is to be used in the RRC connected statefrom the BS through the UE dedicated configuration. When the UE isconfigured to perform the D2D transmission and reception only in the RRCconnected state, the UE should establish the RRC connection when tryingto perform the D2D transmission and reception. Then, considering themobility in the RRC connected state, the UE has the burden to performall procedures that should be performed in the RRC connected state, suchas the measurement and the measurement report to a BS depending on theconfiguration, the scheduling monitoring of BS, and so on. It isrequired for a UE in the RRC idle state efficiently perform the D2Doperation by broadcasting the D2D resource information within thecoverage of cell, and also required to clearly regulate the D2Doperation of UE according to the resource broadcasted when the RRC idlestate of UE is switched to the RRC connected state.

Hereinafter, mode 1 is to schedule the resource for the D2D signaltransmission performed by a UE, and mode 2 is to select a specificresource for the D2D signal transmission within the resource poolperformed by a UE. That is, mode 1 may not allow the degree of freedomof UE or allow at a low level since the D2D signal transmission of UE iscontrolled by a network, and mode 2 may allow the degree of freedom ofUE since the UE receives the configuration or selects a specificresource by itself within a predetermined resource pool and performs theD2D signal transmission.

The transmission of D2D signal by mode 1 may be simply referred to asmode 1 D2D transmission, and the reception of the D2D signal may beshortly referred to as mode 1 D2D reception. The transmission of D2Dsignal by mode 2 may be simply referred to as mode 2 D2D transmission,and the reception of the D2D signal may be shortly referred to as mode 2D2D reception.

The UE in the RRC idle state may receive the broadcasted resource poolinformation that may be applied to mode 2. The resource pool informationmay represent the resources that may receive the D2D signal transmittedby mode 2. In this sense, the resource pool information may also bereferred to as the reception resource pool information.

In order for the UE that is going to perform the D2D reception toefficiently perform the D2D reception, it is preferable that a BSbroadcasts the resource that is used for the D2D reception. That is, inthe case that the corresponding cell is the cell that supports the D2Doperation of UE, it is preferable that the cell should provide thereception resource that is used for the D2D reception of UE at theleast.

When the UE receives the D2D reception resource in the RRC idle statefrom the cell, the UE determines that the UE may perform the D2Dreception in the RRC idle state using the resource. That is, in thiscase, the UE is not required to perform the RRC connection procedure inorder to enter the RRC connected state only for the D2D reception.

The UE that is performing the D2D reception in the RRC idle state mayperform the RRC connection procedure in order to receive theconfiguration required for the D2D transmission resource and the D2Dtransmission or in order to perform the cellular communication. In thiscase, the UE determines the D2D reception resource received through thebroadcast to be still valid. Accordingly, the UE may continuouslyperform the D2D reception using the valid D2D reception resource, andaccordingly, there is no interruption of D2D reception service accordingto the RRC connection attempt.

The UE determines the D2D reception resource that is received throughbroadcast to be still valid even after entering into the RRC connectedstate by successfully performing the RRC connection procedure.Accordingly, the UE may continuously perform the D2D reception using thevalid D2D reception resource, and accordingly, there is no interruptionof D2D reception service according to entering the RRC connected state.

Until the UE in the RRC connected state receives the D2D receptionresource and/or the D2D reception configuration from the BS through theUE dedicated signaling, the UE may determine the D2D reception resourcereceived through broadcast to be still valid.

When the UE is performing the D2D reception using the D2D receptionresource recognized by the broadcasted information, the UE shouldmaintain the latest system information that includes the informationnotifying the D2D reception resource in the RRC connected state. Thatis, the UE monitors whether the system information including theinformation notifying the D2D reception resource is changed. When the UEis noticed the fact that the system information including theinformation notifying the D2D reception resource is changed, the UEreceives the updated system information and performs the D2D operationaccording to the updated system information.

When the UE receives the broadcasted resource pool information that maybe applied to mode 2, the UE may apply the resource pool information tothe RRC connected state. When the UE in the RRC idle state receives thebroadcasted resource pool information that may be applied to mode 1, theUE may also apply the resource pool information to the RRC connectedstate. In this case, the resource pool information may represent theresources that may receive the D2D signal, which is transmitted byanother UE in mode 1. In this sense, the resource pool information maybe referred to as the reception resource pool information.

In the description above, the reception resource pool information isdescribed by distinguishing it as modes 1 and 2, but not limitedthereto. That is, the reception resource pool information may indicatethe resource that may be used for the D2D signal reception regardless ofmodes 1 and 2.

The UE in the RRC idle state may receive the broadcasted resource poolinformation that may be applied to mode 2. The resource pool informationmay represent the resources that another UE may transmit the D2D signalby mode 2. In this sense, the resource pool information may be referredto as the transmission resource pool information.

In order for the UE that is going to perform the D2D transmission toperform the D2D transmission more efficiently, the BS may broadcast theresource that is used for the D2D transmission.

Meanwhile, when the UE that uses the network resource performs the D2Dtransmission, it may interfere in the network. In addition, for thepurpose of figuring out the position of the UE, etc., the policy may berequired that the network should perform the D2D transmission only afterthe UE enters the RRC connected state and receives the D2D transmissionresource and configuration from the BS. Accordingly, the D2Dtransmission resource pool information may not be broadcasted by thecell that supports the D2D operation.

When the UE in the RRC idle state receives the information/configurationthat represents the D2D transmission resource from the cell, the UEdetermines that the UE may perform the D2D transmission in the RRC idlestate using the D2D transmission resource. That is, in this case, the UEis not required to perform the RRC connection procedure for entering theRRC connected state only for the D2D transmission.

The UE that is performing the D2D transmission in the RRC idle state maystart the RRC connection procedure. In the case of the D2D transmissionthat is different from the D2D reception, it may be preferable that theUE stops the D2D transmission in order to minimize the influence on theRRC connection procedure when the UE tries to perform the RRCconnection. In this case, the UE restarts the D2D transmission only inthe case that the network configures the D2D transmission resource tothe UE after the UE enters the RRC connected state.

When the UE is performing the D2D transmission using the D2Dtransmission resource indicated by the broadcasted information, the UEshould maintain the latest system information that includes theinformation notifying the D2D transmission resource in the RRC connectedstate. That is, the UE monitors whether there is any changes in thesystem information including the information notifying the D2Dtransmission resource. When the UE is noticed the fact that the systeminformation including the information notifying the D2D transmissionresource is changed, the UE receives the updated system information andperforms the D2D operation according to the updated system information.

FIG. 15 illustrates a D2D operation method performed by a UE accordingto an embodiment of the present invention.

Referring to FIG. 15, a UE receives the system information that includesthe reception resource pool information indicating the resources that isallowed for the reception of D2D signal (step, S210). The systeminformation may be broadcasted. For example, the system may bebroadcasted in the cell in which the UE is camped on or tries to campon.

The D2D signal may be the signal for the D2D communication or the D2Ddiscovery. The signal for the D2D discovery may be, for example, thediscovery announcement.

The UE receives the D2D signal in the RRC idle state and the RRCconnected state using the resources indicated by the reception resourcepool information (step, S220).

The UE may receive the system information that includes the receptionresource pool information in the RRC idle state. Even in the case thatthe UE is switched from the RRC idle state to the RRC connected state,the UE may receive the D2D signal using the resources indicated by thereception resource pool information.

The resources indicated by the reception resource pool information maybe used when the UE receives the D2D signal that another UE transmits bymode 1 and/or mode 2.

Hereinafter, each of the steps in FIG. 15 will be described in detail.

The table below represents an example of the system information thatincludes the reception resource pool information indicating that thereception of D2D signal is allowed. The UE that receives the systeminformation may regard that the network (e.g., the E-UTRAN) supports theD2D operation. The system information may configure the resourcesrelated to the D2D communication.

TABLE 2 -- ASN1START SystemInformationBlockType18-r12 ::= SEQUENCE {  commConfig-r12    SEQUENCE {    commRxPool-r12 SL-CommRxPoolList-r12,   commTxPoolNormalCommon-r12 SL-CommTxPoolList-r12  OPTIONAL,  -- NeedOR    commTxPoolExceptional-r12  SL-CommTxPoolList-r12   OPTIONAL,  --Need OR    commSyncConfig-r12    SL-SyncConfigList-r12   OPTIONAL  --Need OR   } OPTIONAL,  -- Need OR   lateNonCriticalExtension OCTETSTRING OPTIONAL,   ... } -- ASN1STOP

Referring to Table 2, in the system information, the ‘commRxPool’indicates the resources that the UE is allowed to receive the D2Dcommunication, that is, the resource pool, in the RRC idle state and theRRC connected state. The ‘commRxPool’ may an example of the receptionresource pool information indicating the resource that is allowed forthe reception of D2D signal, described in FIG. 15. The ‘commSyncConfig’indicates the configuration that the UE is allowed to transmit orreceive the synchronization information. The ‘commTxPoolNormalCommon’indicates the resources (resource pool) that the UE is allowed totransmit the D2D communication during the RRC idle state, or indicatesthe resources that are allowed when the UE performs the D2D transmissionthrough the frequency that is not the primary frequency in the RRCconnected state.

That ‘commTxPoolExceptional’ indicates the resource that is allowed forthe UE to transmit the D2D communication in an exceptional condition.

When the system information is broadcasted, and includes the‘commTxPoolExceptional’, but not includes the ‘commTxPoolNormalCommon’,the UE may transmit the D2D signal (control information or data) for theD2D communication by selecting a specific resource within the resourcepool that is indicated by the ‘commTxPoolExceptional’.

Table 3 below represents another example of the system information thatincludes the reception resource pool information indicating that thereception of D2D signal is allowed.

TABLE 3 -- ASN1START SystemInformationBlockType19-r12 ::= SEQUENCE {  discConfig-r12 SEQUENCE {   discRxPool-r12 SL-DiscRxPoolList-r12,  discTxPoolCommon-r12 SL-DiscTxPoolList-r12     OPTIONAL,  -- Need OR  discTxPowerInfo-r12 SL-DiscTxPowerInfoList-r12   OPTIONAL,  -- Cond Tx  discSyncConfig-r12 SL-SyncConfigList-r12     OPTIONAL  -- Need OR   }OPTIONAL,  -- Need OR   discInterFreqList-r12 SL-CarrierFreqInfoList-r12  OPTIONAL,  -- Need OR   lateNonCriticalExtension OCTET STRING    OPTIONAL,   ... } SL-CarrierFreqInfoList-r12 ::= SEQUENCE (SIZE(1..maxFreq)) OF SL- CarrierFreqInfo-r12 SL-CarrierFreqInfo-r12::=SEQUENCE {   carrierFreq-r12    ARFCN-ValueEUTRA-r9,  plmn-IdentityList-r12 PLMN IdentityList4-r12     OPTIONAL  -- Need OP} PLMN-IdentityList4-r12 ::= SEQUENCE (SIZE (1..maxPLMN-r11)) OF   PLMN-IdentityInfo2-r12 PLMN-IdentityInfo2-r12 ::= CHOICE {   plmn-Index-r9INTEGER (1..maxPLMN-r11),   plmnIdentity-r12 PLMN-Identity } -- ASN1STOP

In Table 3 above, the ‘discInterFreqList’ indicates the neighborfrequencies in which the discovery announcement is supported. The‘discRxPool’ indicates the resources in which the reception of thediscovery signal (e.g., the discovery announcement) is allowed in theRRC idle state and the RRC connected state. The ‘discRxPool’ may anexample of the reception resource pool information indicating that thereception of D2D signal is allowed, described in FIG. 15. The‘discSyncConfig’ indicates the configuration that the UE is allowed totransmit or receive the synchronization information. The‘discTxPoolCommon’ indicates the resources (resource pool) that areallowed for the UE to transmit the discovery signal (e.g., the discoveryannouncement) during the RRC idle state. The ‘plmn-IdentityList’ is thelists of the PLMN IDs. The ‘plmn-Index’ is the index that corresponds tothe entry in the ‘plmn-IdentityList’ field of SIB 1(systeminformationblock type 1).

FIG. 16 exemplifies a D2D operation method of a UE.

Referring to FIG. 16, a network broadcasts the reception resource poolinformation (step, S301). As described above by referring to FIG. 15,the reception resource pool information may be broadcasted with beingincluded in the system information.

When UE 1 is positioned within the network coverage, UE 1 may receivethe reception resource pool information. UE 1 may receive the receptionresource pool information in the RRC idle state.

UE 2 neighboring to UE 1 may transmit the D2D signal to UE 1 (step,S302). UE 1 receives the D2D signal transmitted by UE 2 using theresources indicated by the reception resource pool information (step,S303).

UE 1 may perform the RRC connection configuration/reconfiguration withthe network (step, S304). The procedure is described above by referringto FIGS. 5 and 6.

When the RRC connection configuration/reconfiguration is completed withthe network, UE 1 is in the RRC connected state. UE 1 may receive thededicated signal for UE 1, which indicates the D2D transmission resourcein the RRC connected state (step, S304-1). The dedicated signal mayinclude the information indicating the D2D transmission resource only,not include the information indicating the D2D reception resource.

UE 1 may transmit the D2D signal to UE 2 using the D2D transmissionresource that is configured through the dedicated signal, and receivethe D2D signal from UE 2 using the resources indicated by the receptionresource pool information received in the RRC idle state (step, S306).

Using such a method, the interruption does not occur during thereception of the D2D signal by UE 1. Therefore, the reliability of D2Doperation increases.

Meanwhile, with respect to the UE in the RRC connected state, thenetwork may provide the D2D configuration for the D2D operation in mode1 or mode 2. When the network does not include the resource poolinformation in the D2D configuration, the UE may apply the resource poolinformation that is broadcasted for the D2D operation in the RRCconnected state.

After the UE is configured for the D2D that enables the operation ofmode 1 or mode 2, the UE may experience the radio link failure (RLF),the network coverage breakaway, or the like. In this case, the UE maypreserve the D2D configuration that is previously configured and the RRCconnected mode.

When the time for preserving the D2D configuration and the resource poolinformation is referred to as the preserved time, the UE does not regardthe D2D configuration and the resource pool information as beingapplicable after the preserved time is lapsed.

Otherwise, after the preserved time is lapsed, the UE may remove theresource pool information that was regarded as being applicable.

Or, after the preserved time is lapsed, the UE may perform the D2Dtransmission in mode 1 using the resource pool that is preconfigured.That is, after the preserved time is lapsed, the UE may switch theresource pool to the resource pool that is preconfigured.

The maximum value of the preserved time may be signaled by the networkor predetermined. The preserved time may be started when the connectionproblem such as the RLF or a case of leaving the network coverage isdetected.

<The D2D Communication in the RRC Idle State>

With respect to the D2D transmission within a cell in the RRC idlestate, a network may control whether the D2D transmission is allowed.The network may allow the D2D transmission by the UE in the RRC idlestate within a specific cell, that is, the D2D transmission in mode 2.In this case, the network may notify whether to support the D2Dtransmission in mode 2 to the UE, for example, through the broadcastedsystem information in the specific cell. When the UE fails to receivethe system information, the UE may regard the D2D transmission in theRRC idle state is not allowed within the cell.

In relation to the D2D reception within the cell in the RRC idle state,as far as the D2D signal reception is allowed by the network, thenetwork is not required to control the D2D signal reception. That is,whether to receive the D2D signal may be determined by the UE.Regardless of whether to support the D2D transmission in the RRC idlestate within a specific cell, the UE may receive the D2D signal.

<The D2D Communication in the RRC Connected State>

When a UE is in the RRC connected state, the D2D transmission of the UEis allowed only in the case that the UE has the valid D2D configurationthat may be applicable in the RRC connected state. For this reason, anetwork may provide the D2D configuration for the UE through the RRCconnection reconfiguration message that includes the D2D configuration.

That is, the UE in the RRC connected state is allowed to perform the D2Dtransmission only in the case that the network provides the D2Dconfiguration for the UE. The D2D configuration may be provided throughthe dedicated signal for the UE.

The reception of D2D signal in the RRC connected state may be determinedby the UE since the network allows the D2D signal. That is, thereception of the D2D signal is allowed regardless of whether the UE isprovided with the D2D configuration through the dedicated signal.

<Mode Configuration>

A network may configure for the UE which mode is available to operate orwhich mode should be appropriate to operate from mode 1 or mode 2, andit is referred to as the mode configuration. In this case, the signalingfor the mode configuration may use the high layer signal such as the RRCor the low layer signal such as the physical layer signal. Since themode configuration is not performed frequently nor sensitive to thelatency, the RRC signal may be used.

Only mode 2 may be applied to the UE in the RRC idle state. On the otherhand, both of mode 1 and mode 2 may be applied to the UE in the RRCconnected state. That is, the selection/configuration between mode 1 andmode 2 is required only for the UE in the RRC connected state.Accordingly, the dedicated RRC signaling may be used for the modeconfiguration.

Meanwhile, in the mode configuration, the available option is to beselected one between mode 1 and mode 2, or to be selected among mode 1,mode 2 and mode 1 & 2. When mode 1 & 2 is configured, the networkschedules the resource for the D2D transmission, and the UE may performthe D2D transmission using the scheduled resource. In addition, the UEmay also perform the D2D transmission by selecting a specific resourcewithin the resource pool.

The network may configure one of mode 1, mode 2 and mode 1 & 2 to the UEthrough the dedicated RRC signaling.

<Resource Pool Configuration and Signaling>

In considering the aspect of the D2D signal transmission of UE, when theUE configured in mode 1 performs the D2D transmission, the UE isscheduled with the resource for the D2D transmission from a network.Accordingly, the UE is not required to know the resource pool for theD2D transmission. When the UE configured in mode 2 performs the D2Dtransmission, the UE should know the resource pool for the D2Dtransmission.

In considering the aspect of the D2D signal reception of UE, in orderfor a UE to receive the D2D transmission in mode 1 by another UE, the UEshould know the mode 1 reception resource pool. Herein, the mode 1reception resource pool may be the union of resource pools that is usedfor the D2D transmission in mode 1 of a serving cell and a neighboringcell. In order for the UE to receive the D2D transmission in mode 2 byanother UE, the UE should know the mode 2 reception resource pool.Herein, the mode 2 reception resource pool may be the union of resourcepools that is used for the D2D transmission in mode 2 of a serving celland a neighboring cell.

For the resource pool 1 of mode 1, the UE is not required to know themode 1 transmission resource pool. This is because the mode 1 D2Dtransmission is scheduled by a network. However, in order for a specificUE to receive the mode 1 D2D transmission from other UE, the specific UEshould know the mode 1 transmission resource pool of other UEs. In orderfor the specific UE to receive the mode 1 D2D transmission in the RRCidle state, it may be required for a cell to broadcast the informationindicating the mode 1 reception resource pool. This information may beapplicable for both of the RRC idle state and the RRC connected state.

When a specific cell wants to allow the mode 1 D2D reception for the UEin the cell, the information indicating the mode 1 reception resourcepool may be broadcasted. The mode 1 reception resource pool informationmay be applicable for both of the RRC idle state and the RRC connectedstate.

In order that the mode 2 D2D transmission is allowed/enabled for the UEin the RRC idle state, it is required to notify the resource pool thatis available to use for the mode 2 D2D transmission in the RRC idlestate to the UE. For this, the cell may broadcast the resource poolinformation. That is, when a specific cell wants to allow the D2Dtransmission for the UE in the RRC idle state, the specific cell maybroadcast the resource pool information that represents the resourcepool that may be applied to the D2D transmission in the RRC idle statethrough the system information.

Similarly, in order that the mode 2 D2D reception is allowed/enabled forthe UE in the RRC idle state, it is required to notify the resource poolthat is available to use for the mode 2 D2D reception in the RRC idlestate to the UE. For this, the cell may broadcast the reception resourcepool information that represents the reception resource pool.

That is, when a specific cell wants to allow the D2D reception by the UEin the RRC idle state, the specific cell may broadcast the resource poolinformation that represents the resource pool that may be applied to theD2D reception in the RRC idle state through the system information.

The resource pool information that represents the resource pool that maybe applied to the D2D transmission in the RRC idle state may also beapplied for the mode 2 D2D transmission in the RRC connected state. Whena network configures the mode 2 operation for a specific UE through adedicated signal, it may be implemented to provide the resource poolthat is the same as the broadcasted resource pool. Otherwise, it may beregarded that the broadcasted resource pool may be applicable for bothof the D2D transmission and the D2D reception in the RRC connectedstate. As far as the UE is configured in mode 2, it may be regarded thatthe broadcasted resource pool may be valid in the RRC connected state.That is, so far as other resource is not indicated by the dedicatedsignaling, the broadcasted mode 2 D2D resource pool information may alsobe used for the mode 2 D2D communication.

It is not necessarily required to notify the resource pool informationthrough the dedicated signal for the specific UE in the networkcoverage. In the case of notifying the resource pool information throughdedicated signaling, the optimization may be available by decreasing themonitoring resource for the specific UE. However, such an optimizationmay require the complex network cooperation between cells.

FIG. 17 is a block diagram illustrating a UE in which the embodiments ofthe present invention are implemented.

Referring to FIG. 17, a UE 1100 includes a processor 1110, a memory 1120and a radio frequency (RF) unit 1130. The processor 1110 implements theproposed functions, processes and/or methods. For example, the processor1110 may be configured to receive the system information including thereception resource pool information, and to receive the D2D signal usingthe resources indicated by the reception resource pool information. Thereception resource pool information may indicate the resources in whichthe reception of D2D signal is allowed in both of the RRC idle state andthe RRC connected state of the UE.

The RF unit 1130 is connected to the processor 1110 and sends andreceives radio signals.

The processor may include Application-Specific Integrated Circuits(ASICs), other chipsets, logic circuits, and/or data processors. Thememory may include Read-Only Memory (ROM), Random Access Memory (RAM),flash memory, memory cards, storage media and/or other storage devices.The RF unit may include a baseband circuit for processing a radiosignal. When the above-described embodiment is implemented in software,the above-described scheme may be implemented using a module (process orfunction) which performs the above function. The module may be stored inthe memory and executed by the processor. The memory may be disposed tothe processor internally or externally and connected to the processorusing a variety of well-known means.

What is claimed is:
 1. A method for a device-to-device (D2D) operationin a wireless communication system, the method performed by a userequipment (UE) and comprising: receiving system information includingreception resource pool information and exceptional resourceinformation; and communicating with another UE using resources informedby the system information, wherein the reception resource poolinformation informs resources in which reception of a D2D signal isallowed while in a radio resource control (RRC) idle state and while inan RRC connected state, wherein, when the UE receives the systeminformation in the RRC idle state, the UE receives the D2D signal usingthe resources informed by the reception resource pool information evenin a case that the UE is switched from the RRC idle state to the RRCconnected state, and wherein, the UE transmits another D2D signal usingresources informed by the exceptional resource information when the UEdetects a radio link failure at a cell providing the system information.2. The method of claim 1, wherein the system information is broadcasted.3. The method of claim 1, wherein the D2D signal is a signal for a D2Dcommunication or a signal for a D2D discovery.
 4. A user equipment (UE),the UE comprising: a transceiver configured to transmit or receive aradio signal; and a processor operatively connected to the transceiver,wherein the processor is configured to: receive system informationincluding reception resource pool information and exceptional resourceinformation; and communicate with another UE using resources informed bythe system information, wherein the reception resource pool informationinforms resources in which reception of a D2D signal is allowed while ina radio resource control (RRC) idle state and while in an RRC connectedstate, wherein, when the UE receives the system information in the RRCidle state, the UE receives the D2D signal using the resources informedby the reception resource pool information even in a case that the UE isswitched from the RRC idle state to the RRC connected state, andwherein, the UE transmits another D2D signal using resources informed bythe exceptional resource information when the UE detects a radio linkfailure at a cell providing the system information.
 5. The UE of claim4, wherein the system information is broadcasted.
 6. The UE of claim 4,wherein the D2D signal is a signal for a D2D communication or a signalfor a D2D discovery.